On-the-fly laser beam path error correction for specimen target location processing

ABSTRACT

A laser-based workpiece processing system includes sensors connected to a sensor controller that converts sensor signals into focused spot motion commands for actuating a beam steering device, such as a two-axis steering mirror. The sensors may include a beam position sensor for correcting errors detected in the optical path, such as thermally-induced beam wandering in response to laser or acousto-optic modulator pointing stability, or optical mount dynamics.

RELATED APPLICATIONS

This is a continuation of U.S. patent application Ser. No. 10/985,840,filed Nov. 9, 2004, now U.S. Pat. No. 7,245,412, which is acontinuation-in-part of U.S. patent application Ser. No. 10/077,691,filed Feb. 15, 2002, now U.S. Pat. No. 6,816,294, which claims benefitof U.S. Provisional Patent Application No. 60/269,646, filed Feb. 16,2001.

TECHNICAL FIELD

This invention relates to laser processing of integrated circuit (“IC”)links and, in particular, to a laser system and method employing a laserbeam and substrate positioning system that incorporates a steeringmirror to compensate for stage positioning errors and enhance linksevering throughput.

BACKGROUND OF THE INVENTION

Yields in IC device fabrication processes often incur defects resultingfrom alignment variations of subsurface layers or patterns orparticulate contaminants. FIGS. 1, 2A, and 2B show repetitive electroniccircuits 10 of an IC device or workpiece 12 that are typicallyfabricated in rows or columns to include multiple iterations ofredundant circuit elements 14, such as spare rows 16 and columns 18 ofmemory cells 20. With reference to FIGS. 1, 2A, and 2B, circuits 10 arealso designed to include particular laser severable circuit links 22between electrical contacts 24 that can be removed to disconnect adefective memory cell 20, for example, and substitute a replacementredundant cell 26 in a memory device such as a DRAM, an SRAM, or anembedded memory. Similar techniques are also used to sever links toprogram a logic product, gate arrays, or ASICs.

Links 22 are designed with conventional link widths 28 of about 2.5microns, link lengths 30, and element-to-element pitches(center-to-center spacings) 32 of about 2 microns or less from adjacentcircuit structures or elements 34, such as link structures 36. Linkdimensions and pitches are continually being reduced by devicemanufacturers. Although the most prevalent link materials have beenpolysilicon and like compositions, memory manufacturers have morerecently adopted a variety of more conductive metallic link materialsthat may include, but are not limited to, aluminum, copper, gold nickel,titanium, tungsten, platinum, as well as other metals, metal alloys suchas nickel chromide, metal nitrides such as titanium or tantalum nitride,metal silicides such as tungsten silicide, or other metal-likematerials.

Circuits 10, circuit elements 14, or cells 20 are tested for defects.The links to be severed for correcting the defects are determined fromdevice test data, and the locations of these links are mapped into adatabase or program. Laser pulses have been employed for more than 20years to sever circuit links 22. FIGS. 2A and 2B show a laser spot 38 ofspot size diameter 40 impinging a link structure 36 composed of a link22 positioned above a silicon substrate 42 and between component layersof a passivation layer stack including an overlying passivation layer 44(shown in FIG. 2A but not in FIG. 2B) and an underlying passivationlayer 46 (shown in FIG. 2B but not in FIG. 2A). FIG. 2C is a fragmentarycross-sectional side view of the link structure of FIG. 2B after thelink 22 is removed by the laser pulse.

FIG. 3 is a plan view of a beam positioner travel path 50 performed by atraditional link processing positioning system. Because links 22 aretypically arranged in rows 16 and columns 18 (representative ones shownin dashed lines), the beam position and hence the laser spots 38 arescanned over link positions along an axis in a first travel direction52, moved to a different row 16 or column 18, and then scanned over linkpositions along an axis in a second travel direction 54. Skilled personswill appreciate that scanning may include moving the workpiece 12,moving the laser spot 38, or moving the workpiece 12 and the laser spot38. Skilled persons will also appreciate that many different linklayouts are possible and that FIG. 3 is merely a representative layout.

Traditional positioning systems are characterized by X-Y translationtables in which the workpiece 12 is secured to an upper stage that movesalong a first axis and is supported by a lower stage that moves along asecond axis that is perpendicular to the first axis. Such systemstypically move the workpiece relative to a fixed beam position or laserspot 38 and are commonly referred to as stacked stage positioningsystems because the lower stage supports the inertial mass of the upperstage which supports workpiece 12. These positioning systems haveexcellent positioning accuracy because interferometers are typicallyused along each axis to determine the absolute position of each stage.This level of accuracy is preferred for link processing because thelaser spot size 40 is typically only a little bigger than link width 28,so even a small discrepancy between the position of laser spot 38 andlink 22 can result in incomplete link severing. In addition, the highdensity of features on semiconductor wafers results in small positioningerrors potentially causing laser damage to nearby structures. Stackedstage positioning systems are, however, relatively slow because thestarting, stopping, and change of direction of the inertial mass of thestages increase the time required for the laser tool to process all thedesignated links 22 on workpiece 12.

In split-axis positioning systems, the upper stage is not supported by,and moves independently from, the lower stage and the workpiece iscarried on a first axis or stage while the tool, such as a fixedreflecting mirror and focusing lens, is carried on the second axis orstage. Split-axis positioning systems are becoming advantageous as theoverall size and weight of workpieces 12 increase, utilizing longer andhence more massive stages.

More recently, planar positioning systems have been employed in whichthe workpiece is carried on a single stage that is movable by two ormore actuators while the tool remains in a substantially fixed position.These systems translate the workpiece in two dimensions by coordinatingthe efforts of the actuators. Some planar positioning systems may alsobe capable of rotating the workpiece.

Semiconductor Link processing (“SLP”) systems built by ElectroScientific Industries, Inc. (“ESI”) of Portland, Oreg. employ on-the-fly(“OTF”) link processing to achieve both accuracy and high throughput.During OTF processing, the laser beam is pulsed as a linear stage beampositioner passes designated links 12 under the beam position. The stagetypically moves along a single axis at a time and does not stop at eachlink position. The on-axis position of beam spot 38 in the directiontravel 52 does not have to be accurately controlled; rather, itsposition is accurately sensed to trigger laser spot 38 to hit link 22accurately.

In contrast and with reference again to FIG. 3, the position of beamspot 38 along cross-axes 56 or 58 is controlled within specifiedaccuracy as the beam positioner passes over each link 22. Due to theinertial mass of the stage or stages, a set-up move to start an OTF runproduces ringing in the cross-axis position, and the first link 22 in anOTF run cannot be processed until the cross-axis position has settledproperly. The settling delay or setting distance 60 reduces processingthroughput. Without a settling delay (or, equivalently, a buffer zone ofsettling distance 60) inserted before the first laser pulse, severallinks 22 would be processed with serious cross-axis errors.

Although OTF speed has been improved by accelerating over gaps in thelink runs, one limiting factor on the effectiveness of this “gapprofiling” is still the requirement for the cross axis to settle withinits specified accuracy. At the same time, feature sizes, such as linklength 30 and link pitch 32, are continuing to decrease, causing theneed for dimensional precision to increase. Efforts to further increasethe performance of the stage or stages substantially increase the costsof the positioning system.

The traditional way to provide for two-axis deflection of a laser beamemploys a high-speed short-movement positioner (“fast positioner”) 62,such as a pair of galvanometer driven mirrors 64 and 66 shown in FIG. 4.FIG. 4 is a simplified depiction of a galvanometer-driven X-axis mirror64 and a galvanometer-driven Y-axis mirror 66 positioned along anoptical path 70 between a fixed mirror 72 and focusing optics 78. Eachgalvanometer-driven mirror deflects the laser beam along a single axis.U.S. Pat. No. 4,532,402 of Overbeck discloses a stacked stage beampositioning system that employs such a fast positioner, and U.S. Pat.Nos. 5,751,585 and 5,847,960 of Cutler et al. disclose split-axis beampositioning systems in which the upper stage(s) carry at least one fastpositioner. Systems employing such fast positioners are used for nonlinkblowing processes, such as via drilling, because they cannot currentlydeliver the beam as accurately as “fixed” laser head positioners.

The split-axis nature of such positioners may introduce rotational Abbeerrors, and the galvanometers may introduce additional positioningerrors. In addition, because there must be separation between the twogalvanometer-controlled mirrors, the mirrors cannot both be located nearthe entrance pupil to the focusing optics. This separation results in anoffset of the beam that can degrade the quality of the focused spot.Moreover, two-mirror configurations constrain the entrance pupil to bedisplaced farther from the focusing optics, resulting in an increasedcomplexity and limited numerical aperture of the focusing optics,therefore limiting the smallest achievable spot size. Even assuming suchpositioners could be used for link-severing, the above-described spotquality degradation would cause poor quality link-severing or incompletelink-severing and result in low open resistance across severed links 22.

What is still needed, therefore, is a system and method for achievinghigher link-processing throughput while maintaining focused spotquality.

SUMMARY OF THE INVENTION

An object of the invention is, therefore, to provide a system and/ormethod for achieving higher link-processing throughput while maintainingfocused spot quality.

Another object of the invention is to employ a two-axis steering mirrorto correct for linear stage settling errors.

A further object of the invention is to provide a positioner systememploying coordinated motion for semiconductor link processingapplications.

This invention preferably employs a two-axis steering mirror, pivotallymounted at the entrance pupil of the focusing lens, to performsmall-angle motions that deflect the laser beam enough to compensate forcross-axis settling errors on the order of tens of microns. Although thesettling errors occur in both axes, one embodiment of this invention isconcerned primarily with correcting settling errors in a cross-axisdirection to the OTF direction of linear stage travel. A two-axissteering mirror is employed for these corrections because either axis ofthe linear stage may be used as the OTF axis. The beam steering mirroris preferably used for error correction only and does not requirecoordination with or modification of the linear stage position commands,although such coordination is possible. The steering mirror may also beemployed to correct for various sensed system errors, such as thermaldrift, optical distortions, and optical path device errors that lead tolaser beam wandering.

At least three technologies can be used to tilt a mirror in two axesabout a single pivot point. Two of these technologies create a faststeering mirror (FSM) using voice coil actuators or piezoelectricactuators to tilt a mirror. Piezoelectric FSMs are preferred. Deformablemirrors represent a third technology and employ piezoelectric orelectrostrictive actuators to deform the surface of a mirror. Other beamsteering technologies can be used to tilt a mirror in practice of theinvention.

Advantages of the invention include the elimination of cross-axissettling time, resulting in increased throughput particularly for SLPsystems. The invention also facilitates improved manufacturability ofthe main positioning stage(s) due to relaxed servo performancerequirements because the steering mirror can correct for linear stageerrors.

Additional objects and advantages of this invention will be apparentfrom the following detailed description of preferred embodiments thereofwhich proceed with reference to the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic diagram of a portion of a DRAM showing theredundant layout of and programmable links in a spare row of genericcircuit cells.

FIG. 2A is a fragmentary cross-sectional side view of a conventional,large semiconductor link structure receiving a laser pulse characterizedby prior art pulse parameters.

FIG. 2B is a fragmentary top view of the link structure and the laserpulse of FIG. 2A, together with an adjacent circuit structure.

FIG. 2C is a fragmentary cross-sectional side view of the link structureof FIG. 2B after the link is removed by the prior art laser pulse.

FIG. 3 is a plan view of a prior art beam travel path.

FIG. 4 is a simplified side view of a prior art fast positioneremploying a pair of galvanometer-driven mirrors that deflect the laserbeam along different respective single axes.

FIG. 5 schematically illustrates a side sectional view of a preferredtwo-axis mirror employed in the practice of the invention.

FIG. 6 schematically illustrates a partial front view of a preferredtwo-axis mirror employed in the practice of the invention.

FIG. 7 illustrates the effect of the steering mirror during the OTF run.

FIG. 8 illustrates an exemplary multi-row, cross-axis dithering(“MRCAD”) operational mode work path.

FIG. 9 is a side sectional view of a representative two-axis steeringmirror.

FIG. 10 is a simplified plan view of a representative two-axis steeringmirror.

FIG. 11 is a simplified schematic block diagram of an exemplarypositioner control system for coordinating the stage positioning and thesteering mirror for error correction.

FIG. 12 is a simplified schematic block diagram of an exemplarypositioner control system for coordinating the stage positioning and thesteering mirror for beam-to-work scans and error correction.

FIG. 13 illustrates a first alternative MRCAD operational mode work pathin which the non-deflected laser beam pathway runs between adjacent rowsof links.

FIG. 14 illustrates a second alternative MRCAD operational mode workpath in which the non-deflected laser beam pathway cross-axis positionis adjusted as its on-axis motion progresses along selected link rows.

FIG. 15A illustrates a third alternative MRCAD operational mode workpath in which the non-deflected laser beam path cross-axis position isadjusted to coincide with the desired link blow position as the on-axismotion of the laser beam progresses along selected link rows.

FIG. 15B illustrates a fourth alternative MRCAD operational mode workpath in which shorter link runs are nested within longer link runs.

FIG. 16 illustrates a fifth alternative MRCAD operational mode work pathin which the effective size of a gap in a link run can be extended byprocessing a portion of first link run in a second link run.

FIG. 17 illustrates a sixth alternative MRCAD operational mode work pathin which end links of a link run are processed during processing of anadjacent link run.

FIG. 18 illustrates a seventh alternative MRCAD operational mode workpath in which a first link run is combined with a second link run.

FIG. 19 is a simplified schematic block diagram representing thepositioner control system of FIG. 12, and further including sensors anda sensor controller for correcting various sensed beam position errors.

FIG. 20 is a simplified schematic block diagram representing a portionof the positioner control system of FIG. 11, further including sensorsand a sensor controller for correcting various sensed beam positionerrors.

FIG. 21 is a simplified schematic block diagram representing a lenstranslation sensor coupled to the sensor controller of FIGS. 19 and 20for correcting laser beam deflection errors caused by various Z-axispositions of the lens.

FIG. 22 is a simplified side view of a fast positioner employing a pairof galvanometer-driven mirrors in association with one or two relaylenses to, respectively, image a mirror pivot point at or near thesurface of the other mirror or image both mirror pivot points at or nearthe entrance pupil of a focusing lens.

FIGS. 23-26 are simplified schematic block diagrams representing fourembodiments of laser beam position correction systems of this invention.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

One embodiment of a representative beam positioning system is describedin detail in U.S. Pat. No. 4,532,402 of Overbeck, which is assigned tothe assignee of this application. A preferred X-Y stage is a “Dynamix”model available from Newport Corporation of Irvine, Calif.

The beam positioning system preferably employs a laser controller thatcontrols a stacked, split-axis, or planar positioner system andcoordinates with reflectors to target and focus laser system output to adesired laser link 22 on IC device or workpiece 12. The beam positioningsystem permits quick movement between links 22 on the same or differentworkpieces 12 to effect unique link-severing operations based onprovided test or design data. The beam positioning system mayalternatively or additionally employ the improvements, beam positioners,or coordinated motion schemes described in U.S. Pat. Nos. 5,751,585,5,798,927, and 5,847,960 of Cutler et al., which are assigned to theassignee of this application. Other fixed head or linear motor drivenconventional positioning systems could also be employed, as well as thesystems employed in the 9000, 9800, and 1225 model series manufacturedby ESI of Portland, Oreg., the assignee of this patent application.

With reference to FIGS. 5 and 6 and with respect to this invention, thefinal turn mirror of a fixed head system or alternatively fastpositioner 66 (FIG. 4) is preferably replaced by a single high-speed,high-accuracy two-axis steering mirror system 100 that includes a mirror102 capable of actuation with at least two degrees of freedom. Mirror102 has a centrally positioned pivot point 104 that preferably coincideswith an entrance pupil 106 of a focusing lens 108. Two-axis steeringmirror system 100 is preferably used for error correction, although itmay be employed for beam steering because either axis of the linearstage may be used as the OTF axis.

Because the beam is focused to a very fine spot size for SLPapplications, the mechanism directing mirror system 100 preferablypivots the mirror 102 along at least two axes about pivot point 104,which is located at or near the entrance pupil of focusing optics orlens 108. Small angle perturbations in the position of mirror 102deflect the beam enough to correct for linear stage settling errors atthe work surface, and because mirror 102 is located at or near theentrance pupil of focusing lens 108, the beam is shifted withoutdistorting the focused spot, allowing delivery of a small, high qualityspot.

In one embodiment, settling errors in a cross-axis direction 110 arecorrected by mirror 102, while motion in an on-axis direction 112 is notcorrected. This single axis correction allows the linear stageinterferometer feedback to be the sole source of laser pulse triggering.However, with proper coordination, on-axis direction 112 steering mirror102 motion is possible, although it complicates the design andintroduces additional error sources that can degrade on-axis direction112 accuracy if such errors are not addressed.

Motion in each axis of mirror 102 exhibits scale factor and offseterrors, noise, and cross-axis coupling. These error sources can bewell-controlled and calibrated out in the system, with noise andtemperature stability effects controlled through conventional designtechniques.

Calibration of mirror system 100 through beam-to-work (“BTW”) alignmentscan correct for any non-linearity and alignment errors in steeringmirror 102. Traditionally, the term beam-to-work is used as nomenclaturefor the process of scanning the linear stage back and forth, whiledirecting the laser beam spot at low power at an alignment target on thewafer or workpiece 12 (FIG. 1). Optical measurements of the reflectionoff the target are used to precisely determine target and hence waferlocation. By scanning several targets with BTW scans, the offset androtation of the wafer relative to the beam spot can be ascertained. Itis also possible to map out other effects such as axis orthogonality andpositional distortions.

After mirror system 100 is added to the laser system, traditional BTWtype scans can be used to map out any inaccuracies/nonlinearities insteering mirror 102 response. This is accomplished by doing a BTW scanwith mirror 102 in the nominal zero offset (in either axis) position.Then mirror 102 is tilted, and another BTW scan is performed todetermine how much lateral offset of the laser beam spot is imparted bythe tilt. By measuring the offset caused by numerous mirror tilts in theU and V axes, mirror system 100 can be fully characterized.

Once the response of mirror system 100 is determined to sufficientlyfine precision, then instead of moving the linear stage back and forth,it is possible to use mirror system 100 for subsequent BTW typealignment scans.

FIG. 7 illustrates the corrective effect of two-axis steering mirrorsystem 100 during an OTF run. A linear stage ringing is represented by aringing curve 120. Mirror 102 deflects the laser beam in cross-axisdirection 110 as represented by a correction curve 122 that is theinverse of ringing curve 120. The resulting beam position is the sum ofthe linear stage motion and the deflected beam position and isrepresented by a resulting beam path curve 124, which is free fromcross-axis error.

FIG. 8 illustrates using steering mirror system 100 for MRCAD processingduring boustrophedon or raster scanning in the context of link severingto further improve the speed at which links are blown. In a preferredmode of operation, MRCAD scanning is done in cross-axis direction 110while moving along a row 130 of links 132. In preferred embodiments,MRCAD scanning employs steering mirror 102 (FIGS. 5 and 6) to direct thelaser beam along a pathway 134 at links 132 and adjacent links 136 inadjacent rows 138 without needing to move the slower linear motion stagein cross-axis direction 110. This is possible because not all the linksin each row need to be blown. Link processing becomes far more efficientwith MRCAD because the linear or stages do not have to be scanned orslewed down each row, so the total number of link row scans can besubstantially reduced. As integration increases and link sizes, spotsizes, and pitch distance decrease, MRCAD scanning will become an evenmore valuable technique.

In another mode, supplemental on-axis dithering (“SOAD”) uses mirror 102to deflect the beam in on-axis direction 112 (FIGS. 5-7). In thisoperational mode, the beam can be quickly directed ahead in on-axisdirection 112, severing links while the linear motion stage catches up.The SOAD scan ahead or scan behind the stage feature allows thepositioning system to reduce stage velocity changes or to sever severallinks during a single slowed movement segment.

As indicated above, there are at least three technologies that can beemployed to tilt mirror 102 in two axes about pivot point 104. The FSMs,which are preferred, use voice coil actuators or piezoelectric actuatorsto tilt the surface of mirror 102. Suitable voice coil actuated FSMs areavailable from Ball Aerospace Corporation of Broomfield, Colo. andNewport Corporation of Irvine, Calif. However, the preferred actuator isa model S-330 Ultra-Fast Piezo Tip/Tilt Platform manufactured by PhysikInstrumente (“PI”) GmbH & Co. of Karlsruhe, Germany.

Traditional galvanometers are not typically used for this applicationbecause they each tilt a mirror about only one axis and ordinarily haveinsufficient positioning accuracy. Moreover, a pair of physicallyseparated galvanometer mirrors is required for two axes of actuation.This separation is incompatible with the desire that actuation occurabout one pivot point located near the entrance pupil of focusing lens108 (FIGS. 5 and 6) to maintain a high quality laser spot at the surfaceof workpiece 12. Nevertheless, it is possible to employ galvanometerdeflected mirrors in this invention, particularly if employed insingle-axis and small deflection applications to maintain accuracy andwell focused laser spots.

By way of example only, FIGS. 9 and 10 show an FSM two-axis mirrorsystem 200 in which four electrical to mechanical vibration generatorsor transducers are supported by a transducer support platform 220 in aquadrature relationship, whereby transducers 222, 224, 226, and 228 arepositioned at 0, 90, 180, and 270 degrees with respect to a central axis230; therefore, adjacent ones of the transducers are set at right angleswith respect to each other. A movable mirror support member 232 has acentral portion or hub 234 bearing a mirror or reflective surface 236centered with respect to axis 230. Mirror 236 has a diameter of about 30mm or less to reduce its weight and facilitate high frequency responsefor desired beam correction. Mirror 236 is coated with conventionallaser optical coatings to account for laser wavelength or designparameters.

Four lightweight rigid struts or elongated members 242, 244, 246, and248 extend radially from hub 234 of mirror support member 232, and haverespective peripheral terminal portions 252, 254, 256, and 258 affixedto respective transducers 222, 224, 226, and 228, which are electricallymovable voice coils. For a further description of a suitableconventional voice coil/loudspeaker arrangement, see Van Nostrand'sScientific Encyclopedia, Sixth Edition, page 1786. The use of suchconventional loudspeaker coils for the transducers to perform mechanicalactuation, decreases the manufacturing cost of the apparatus. Thefloating mirror support 232 can beneficially be made of a lightweightmaterial, such as metal (e.g. aluminum or beryllium) or plastic,enabling rapid response to the electrical input signals to the voicecoils to be described.

A tip control generator 260 is connected to transducers 224 and 228 tocause them to move in a complementary “push pull” relationship with eachother. Similarly, a tilt control generator 262 is connected totransducers 222 and 226 to cause these coils to also move in acomplementary push pull relationship with each other. A laser beam 270is reflected off reflective surface 236 and a reflected beam 272 ispositioned by the generators controlling the cross axis, which isperpendicular to OTF direction of travel, to compensate for cross axiserrors. The pairs of signals produced by each generator assume apush-pull relationship, so that when transducer 222 is pulling upperterminal portion 252 of support member 232 to the right in FIG. 10,lower transducer 226 is pushing terminal portion 256 to the left, totilt reflective surface 236, thereby deflecting reflected beam 272. Theactuation can be alternated at the beginning of an OTF run, for example,to move reflective surface 236 at a proper frequency and dampedamplitude to compensate for linear stage ringing in cross-axis direction110, thereby eliminating the negative effects of linear stage settlingtime and producing a relatively straight beam path. Thus, links thatwould otherwise be in the conventional buffer zone can be processedaccurately.

Mirror systems suitable for use with this invention can be implementedwith a large enough field to do MRCAD scans by providing beam deflectionin a range of about 50 to 100 microns; however, such mirror systems canalso be implemented for cross-axis correction only by providing beamdeflection in a range of about 10 to 50 microns or as little as about 10to 20 microns. The mirror is preferably positioned within about plus orminus 1 mm of the entrance pupil of the focusing lens. These ranges areexemplary only and can be modified to suit the system design andparticular link processing applications.

The preferred model S-330 Tip/Tilt Platform manufactured by PI usespiezoelectric actuators for high speed, two-dimensional mirror tilting.Strain gage sensors accurately determine mirror position and providefeedback signals to the control electronics and drive circuitry. A morecomplete description of the model S-330 Tip/Tilt Platform is availableat the PI web site, www.physikinstrumente.com.

The main advantages of the PI Piezo Tip/Tilt Platform are that thedevice is commercially available and has a very compact size thatreadily mounts in an ESI model 9820 positioning system.

A disadvantage of the PI Piezo Tip/Tilt Platform is that its beamdeflection range limits its use in the MRCAD application of FIG. 8, eventhough the beam deflection range is sufficient for error correctionapplications. Specifically, the PI Piezo Tip/Tilt Platform is capable ofa “⊥” 5 micron beam deflection range. Positioning the linear stage inthe cross-axis direction to enable the steering mirror to process onerow of links with a +4 micron deflection and then positioning the linearstage to process an adjacent row of links with a −4 micron deflectionwould afford an 8 micron shift of the laser beam in the cross-axisdirection. Combining the operations of the linear stage and steeringmirror in this manner leaves available very little remaining beamdeflection range. Moreover, nonlinear motion, thermal drive, hysteresis,and high-voltage actuation are all inherent problems with piezoelectricactuation that have to be accounted for. Calibration techniques,application of feedback control, and good system design canappropriately manage these challenges.

In addition to all the other above-described advantages, this inventionpermits a relaxation on the requirements for the linear motors perktime, settling time) using the secondary system to correct for errors.This substantially reduces the cost of the linear motors and alsoreduces the dependency of the system throughput on the accelerationlimit of the linear stage or stages.

FIG. 11 shows an embodiment of a positioner control system 300 of thisinvention for coordinating the positioning of X- and Y-axis motionstages 302 and 304, and also the positioning of a two-axis steeringmirror 306 for positioning error correction. Of course, motion stages302 and 304 may be combined into a single planar motion stage havingpositioning control in the X- and Y-axis directions. In a standardoperational mode, two-axis steering mirror 306 is used to correctpositioning errors caused by X- and Y-axis motion stages 302 and 304.

A position command generator 308 generates X- and Y-axis positioncommand signals for delivery through summing junctions 310 and 312 to X-and Y-axis motion controllers 314 and 316 to respective X- and Y-axismotion stages 302 and 304. The actual positions of X- and Y-axis motionstages 302 and 304 are sensed by respective X- and Y-axis positionsensors 318 and 320 and signals representing the actual positions areconveyed to adders or summing junctions 310 and 312 to generate X- andY-axis position error signals. X- and Y-axis motion controllers 314 and316 receive the error signals and act to minimize any errors between thecommanded and actual positions. For high-accuracy applications, X- andY-axis position sensors 318 and 320 are preferably interferometers.

Residual error signals, such as those generated by ringing, are conveyedthrough enabling gates 322 and 324 to a coordinate transformationgenerator 326, which may be optional depending on whether motion stages302 and 304 share a common coordinate system with two-axis steeringmirror 306. In either event, the residual error signals are passedthrough adders or summing junctions 328 and 330 to U- and V-axissteering mirror controllers 332 and 334, which act to tip and/or tiltsteering mirror 306 by controlled amounts to deflect, for example, laserbeam 270 (FIG. 9) to correct for positioning errors of X- and Y-axismotion stages 302 and 304. The actual tip and/or tilt positions oftwo-axis steering mirror 306 are sensed by respective tip and tiltsensors 336 and 338 and signals representing the actual tip and tiltpositions are conveyed to adders or summing junctions 328 and 330 togenerate tip and tilt position error signals. U- and V-axis steeringmirror controllers 332 and 334 receive the error signals and act tocorrect any errors between the commanded and actual positions. Forhigh-accuracy applications, two-axis steering mirror 306 is preferably apiezoelectric tilt/tip platform, and position sensors 318 and 320 arepreferably strain gages. Suitable alternative sensors may includeoptical, capacitive, and inductive sensing techniques. In thisembodiment, skilled workers will understand that U- and V-axis steeringmirror controllers 332 and 334 should be adapted to provide zero to 100volt drive signals to the piezoelectric actuators deflecting two-axissteering mirror 306.

Enabling gates 322 and 324 implement a provision in which positioncommand generator 308 can selectively disable position error correctionfor either the X or the Y axis, thereby enabling error correction forthe cross-axis while leaving the on-axis unaffected, or vice versa.

FIG. 12 shows an embodiment of a positioner control system 340 forcoordinating the positioning of X- and Y-axis motions stages 302 and 304and, in this embodiment, FSM 236 (FIGS. 9 and 10) for MRCAD scans andpositioning error correction. In an extended operational mode, thesteering mirror is used for error correction and MRCAD scanning. In thismode of operation, a position command generator 342 generates X- andY-axis positioning commands for X- and Y-axis motion stages 302 and 304and also U- and V-axis tip and tilt commands for deflecting FSM 236.Summing junctions 328 and 330 generate the positioning command for FSM236 as the sum of the error signals from X- and Y-axis motion stages 302and 304 and, in this embodiment, also the U- and V-axis tip and tiltcommands.

The error signals are generated in the same manner as in the standarderror correction mode. The additional U- and V-axis tip and tiltcommands are produced by position command generator 342 to accomplishthe desired beam-to-work scanning. Because beam-to-work and MRCADapplications typically require wider ranges of mirror deflection, thisembodiment of the invention preferably employs voice coil actuated FSMtwo-axis mirror system 200. However, the following MRCAD applicationsfurther allow using a FSM having a more limited range of mirrordeflection.

The MRCAD link processing application of FIG. 8 shows combining threeadjacent link runs into an MRCAD link run by employing cross-axis laserbeam positioning. In the preferred operating mode, FSM 200 is employedto deflect the laser beam a first link run to laterally spaced link runsbecause it is faster than the linear stage. While the FSM is very fastand is excellent for jumping laterally, it has a limited deflectionrange. Therefore, this invention further includes alternate operationalmodes, some of which also employ the linear stage for cross-axis motion.Other operational modes employ only the linear stage for cross-axismotion.

FIG. 13 shows an operational mode in which the linear stage positions anon-deflected laser beam pathway 350 between row 130 and adjacent row138 of links 132, and employs the FSM to provide all or a portion of thepositioning required to process selected ones of links 132. For example,if the centers of rows 130 and 138 of links are separated by 10 microns,and the FSM has a deflection range of “±” 6 microns, then the FSM can beemployed to very quickly deflect the laser beam to either row 130 oradjacent row 136 for processing links 132 in either row. In thisoperational mode, the FSM not only corrects for linear stage positioningerrors, but also provides lateral (or even on-axis) motion for linkprocessing. Of course, the FSM can execute this mode of link processingwith or without position error correction.

FIG. 14 shows another operational mode in which a non-deflected laserbeam pathway 352 is defined when the linear stage cross-axis position isadjusted as its on-axis motion progresses along rows 130 and 138 oflinks 132. The linear stage cross-axis position may also be changedwhile processing links. The FSM is used to quickly supply any additionallaser beam deflection necessary to properly process links 132 in thedesired cross-axis location in an MRCAD link run.

A link run, as defined by the prior art, has one cross-axis position. AnMRCAD link run is defined as a synthesis of multiple link runs and mayhave numerous cross-axis positions. An MRCAD link run does not reverseon-axis direction but may have any cross-axis move profile, includingreversals.

FIG. 15A shows another operational mode in which a laser beam pathway354 is adjusted in the cross-axis direction using only the linear stageas the on-axis motion progresses along row 356 and adjacent rows 358. Anaspect of this operational mode of the invention is to combine link runsinto an MRCAD link run without an FSM. It is possible to implementnon-deflected laser beam pathway 354 without an FSM. Although the linearmotion stages cannot move the laser beam relative to the workpiece asfast as an FSM, the linear stages can, nevertheless, reduce linkprocessing time and increase throughput.

There are many different ways to combine link runs as defined by theprior art to synthesize an MRCAD link run. Combinations may be betweentwo, three, or more link runs, provided there is sufficient time foreach lateral move. FIG. 15B shows a laser beam pathway 354′ that is thesame as laser beam pathway 354 of FIG. 15A, except for the manner inwhich the laser beam is positioned. Laser beam pathway 354′ represents acombined laser path resulting from FSM deflection either alone or incooperation with cross-axis positioning. FIG. 15B depicts an operationalmode for nesting shorter link runs 358′ within longer link runs 356′. Anon-deflected laser beam pathway 354′ represents the combined motionpath. Without combining link runs, it would be necessary to processlinks along a central linear pathway and perform link runs along theadjacent link runs, as indicated in dash-dot lines. Pathway 354′ ismerely illustrative and may not be followed exactly because of variablelink spacing, row spacing, and other variables. Any pathway may befollowed between links that are to be processed, and any combination ofstage movement and/or steering mirror movements may be used in theprocessing of link runs and MRCAD link runs.

Processing of links 132 (FIG. 14) and links 356 and 358 (FIGS. 15A and15B) is preferably carried out by repetitively performing the followingprocess:

1) Determine whether link runs with a cross-axis separation can becombined and determine an appropriate non-deflected beam pathway for anMRCAD link run.

2) Move to a starting position for an MRCAD link run by executingacceleration, deceleration, stop, and fast settle steps. The time toexecute these steps varies, depending upon the distance traveled.

3) Perform the MRCAD link run. Performing an MRCAD link run entailsramping up to velocity to begin the link run, moving along the desiredpathway to process links, and ramping down and stopping again. Whileperforming prior art link runs or MRCAD link runs, it is preferred toaccelerate, or gap profile, over large gaps, which reduces the timerequired to perform such link runs. Skilled persons will appreciate thatstopping need not take place before the start of a subsequent link runor MRCAD link run. Moreover, MRCAD runs can be intermixed withtraditional prior art link runs. Other processing steps, machiningactivities, or positioner motion may take place between link runs ofeither the prior art or MRCAD type.

If two laterally spaced link runs can be combined, there is an immediatetime savings for each combined link run. However, this time savings isbalanced against time lost if gaps that could be profiled are reduced insize, or if gap profiling becomes impossible due to moving laterally andprocessing links out of a separate parallel link run.

It is possible to transition from one link run to an adjacent link runwhenever the lateral move time is less than or equal to the timerequired to move in the on-axis direction between processed links. Thisis expressed mathematically as:Tlateral move<=Distance on-axis/Vlink run.The time to move laterally varies depending upon the lateral distance,the speed performance parameters of the motion stage and/or steeringmirror, move profiling time, and settling time before reaching the nextlink to be processed.

In another operational mode, it is possible to combine portions ofnearby link runs. FIG. 16 shows an example in which the effective sizeof a gap 360 in a link run 362 can be extended by processing a portion364 of link run 362 in a different, MRCAD link run 366. The overallnumber of link runs is not reduced, but throughput is increased bydecreasing the time to complete link run 362 through the use of gapprofiling applied to one large gap rather than two smaller gaps. Gapprofiling of one large gap is faster because higher peak on-axisvelocity may be reached and there is only one settling event. Byprocessing portion 364 of link run 362 during MRCAD link run 366, theeffective length of gap 360 is increased, whereas without suchlengthening, it may not be possible to perform gap profiling.

FIG. 17 shows another operational mode in which end links 368 of a linkrun 370 are processed during processing of an adjacent, MRCAD link run372. Such processing of end links 368 effectively shortens link run 370,resulting in faster processing of link run 370, which saves time andincreases throughput.

FIG. 18 shows another operational mode in which a link run 374 iscombined with a link run 376 to form an MRCAD link run by startingprocessing in link run 376 and finishing link processing in link run374. In this mode it is not necessary to laterally move back to link run376, which saves time and increases throughput.

The operational modes described with reference to FIGS. 15A, 15B, 16,17, and 18 can be implemented by cross-axis stage positioning and/or FSMdeflection.

The above-described MRCAD operational modes may be applied to motionstages and/or steering mirrors of different performance characteristicsand structures from those described herein.

In typical operation, the steering mirror commands for MRCAD scanningare used to produce cross-axis motion of the laser beam withoutcommanding on-axis motion of the laser beam. However, other applicationsare envisioned that would benefit from on-axis supplemental dithering toboustrophedon scanning.

The invention also facilitates relaxed servo performance requirements ofthe main positioning stages because the steering mirror can correct forlinear stage errors, thermal expansions, optical path errors, andvarious other system errors.

The control schemes depicted in these figures are intended to illustratethe basic implementation and operation of this invention. Skilledpersons will readily appreciate that alternative communication andcontrol structures can be used to practice this invention. Thisinvention further includes more advanced control schemes, such as thoseemploying sensor-based position correction and feedforward commands tothe motion stages and steering mirror.

For example, FIG. 19 shows a group of sensors 380A, 380B, 380C, . . . ,380N (collectively, “sensors 380”) electrically connected to a signalconditioner 382 that performs sensor signal scaling and coordinatetransformations to convert the sensor signals into relative focused spotmotion commands for actuating a beam steering device, such as two-axissteering mirror 306 (FIG. 11) or reflective surface 236 (FIG. 12).Alternatively, signal conditioner 382 may produce steering mirrorcommands directly in any coordinate system without an intermediate stepof producing commands in the coordinate system of the focused spot.Sensors 380 may include sensors for detecting many different phenomenasuch as position, relative position, displacement, velocity, beamposition, beam quality, optical power, temperature, atmosphericpressure, humidity, air movement, and/or turbulence. Position sensorsmay include interferometric, capacitive, inductive, strain, magnetic,and resistive sensors, optical encoders, optical position sensitivedetectors, and CCD arrays. Sensors 380 may include a beam positionsensor for correcting errors detected in the optical path, such asthermally induced beam wandering in response to laser or acousto-opticmodulator (“AOM”) pointing instability, or optical mount dynamics. Othertypes of alternative sensors will be apparent to those skilled in theart.

In the FIG. 19 embodiment, signal conditioner 382 is electricallyconnected to position command generator 342 (FIG. 12), which providespositioning command information and control in positioner control system340 (FIG. 12). Positioner control system 340 may alternatively includeone or more single input/single output controller (SISO) and/or one ormore multiple input/multiple output controller (MIMO). For example, thetwo axes of the linear stage may be controlled by a single MIMOcontroller, rather than two separate SISO controllers. It is alsopossible for a single MIMO controller to control the linear stage axesand the steering mirror axes. Those skilled in the art of feedbackcontrol will understand that there are many different control systemarchitectures and algorithms for controlling multiple actuators inresponse to trajectory commands and sensed signals. The architecturesdepicted here are merely illustrative and are not intended to limit thescope of this invention to a particular architecture or controlmethodology.

FIG. 20 shows a second embodiment, in which signal conditioner 382provides relative X- and Y-axis spot motion control signals to summingjunctions 384 and 386 that combine X- and Y-axis position error signalto provide positioning command information to coordinate transformationgenerator 326 (FIGS. 11 and 12) in positioner control systems 300 (FIG.11) and 340 (FIG. 12).

FIG. 21 shows a particular sensor embodiment, in which a laser beam 388is focused on a workpiece 390 by propagating through a focus lens 392that is translated in Z-axis directions by a lens Z stage 394.Undesirable lateral movement of lens 392 may cause wandering of the X-and Y-axis positions of laser beam 388 on workpiece 390. Undesirablelateral movement may be caused by Z-axis translation of the lens,thermal expansion of system components, or external forces applied tothe optics table or other system components causing excitation ofresonant modes or shifts in compliant structural components. In thisembodiment, sensor 380A is an inductive position sensor that conveys tosignal conditioner 382 a signal proportional to the Z-axis position oflens 392. Signal conditioner 382 includes correction informationrelating the sensed Z-axis positions of lens 392 to associated X- andY-axis beam position errors and can, therefore, provide to thepositioner control system the relative X- and Y-axis spot motion signalsnecessary to correct for the laser spot wandering errors. The correctioninformation is preferably obtained by pre-characterizing the system,such as by measuring laser beam X- and Y-axis deviations as a functionof various Z-axis positions.

The entrance pupil of lens 392 is preferably positioned at or near theactual or virtual reflective surface of a two-axis FSM. A relay lenspositioned before lens 392 may be associated with or positioneddownstream of the FSM. As shown in FIG. 22, the two-axis FSM may bereplaced by a pair of galvanometer-driven mirrors 64 and 66 and mayinclude a relay lens 396 (represented as a single lens component)positioned between them to image the pivot point of mirror 64 at or nearthe surface of mirror 66. An optional relay lens 398 (shown in dashedlines) can be used in cooperation with relay lens 396 to image the pivotpoints of mirrors 64 and 66 at or near the entrance pupil of focusinglens 392. Skilled persons will appreciate that a relay lens positionedin optical association with a two-axis FSM steering mirror may be usedto shift the steering mirror pivot point to a location at or near theentrance pupil of focusing lens 392.

As mentioned above, sensors 380 can be used to provide beam wanderingcorrection signals that are derived from virtually any system-relatedsource. For example, FIG. 23 shows a laser beam position correctionsystem 400 in which a laser 402 emits a laser beam 404 that is deflectedby X- and Y-axis piezoelectric-deflected mirrors 406 and 408 forpropagation through optical path devices 410, such as a variable beamexpander (“VBE”) and an AOM. To reduce laser beam 404 positioning errorscaused by optical path devices 410, a beam splitter 412 reflects aportion of deflected laser beam 404 to a beam position sensor 414. Acontrol system 416 receives from beam position sensor 414 informationrelated to the actual positions of laser beam 404 relative to the idealpositions. Control system 416 uses the information to generate X- andY-axis correction signals for driving X- and Y-axispiezoelectric-deflected mirrors 406 and 408, or alternatively, an FSM,X- and Y-axis galvanometer deflected mirrors, or other beam steeringtechnology. Beam wandering errors caused by optical path devices 410 mayinclude low frequency drift caused by AOM thermal loading, VBEdeflections as a function of beam expansion, and mechanical thermalexpansion. Beam position sensor 414 is preferably a quad photo-detectoror other position-sensitive detector (PSD).

FIG. 24 shows another laser beam position correction system 420, inwhich optical path devices 410 are positioned before X- and Y-axispiezoelectric-deflected mirrors 406 and 408, and in which the systemfurther includes a two-axis FSM 422 that is driven by a control system424 for deflecting laser beam 404 through a focusing lens 426 to aworkpiece 428. In this embodiment, primary and error correctionpositioning of laser beam 404 can be carried out jointly or separatelyby control system 414 or control system 424. It is preferred to separateFSM 422 from the thermal drift correction functions associated withcontrol system 416.

Control system 424 is preferably implemented as a digital signalprocessor having FSM updating rate that is much higher than thepositioning system servo cycle rate. FSM updating is accomplished bycalculating desired XY stage positions at a higher rate than isnecessary for the XY stage, such as by direct computation, orinterpolation of the XY command during the present and subsequent servocycles. The position error is then determined and conveyed to the FSM ata much higher rate.

FIGS. 25 and 26 show alternative laser beam position correction systems430 and 432, in which X- and Y-axis piezoelectric-deflected mirrors 406and 408 are replaced by two-axis FSM 422. Laser beam position correctionsystems 430 and 432 also differ in that optical path devices 410 arepositioned before or after two-axis FSM 422. Of course, depending ontheir functions, some or all of optical path devices 410 may bepositioned in various locations along the path of laser beam 404.

The FSMs employed in this invention preferably have a full-deflectionbandwidth greater than 1001 Hz, and, preferably greater than 1.000 Hz.Their deflection angle is preferably greater than 0.05 milliRadian, andpreferably “±” 8 milliRadians, resulting in a “±” 50 micron displacementof laser beam 404 at the workpiece. Positioning accuracy of laser beam404 is preferably within 50 nanoMeters, and deflection accuracy ispreferably within 10 microRadians.

Skilled workers will appreciate that the two-axis steering mirrorsystems of this invention can be adapted for use in etched-circuit boardvia drilling, micro-machining, and laser trimming applications as wellas for link severing. Moreover, laser-based wafer processing machinespracticing this invention may employ single-axis deflection forprocessing wafer links in one axis direction, after which the wafer isrotated and links are processed in another axial direction, preferablyorthogonal to the first direction.

It will be obvious to those having skill in the art that many changesmay be made to the details of the above-described embodiment of thisinvention without departing from the underlying principles thereof. Thescope of this invention should, therefore, be determined only by thefollowing claims.

1. A method for directing a laser beam toward a target location on aworkpiece in response to target location coordinate position commandinformation, comprising: positioning the workpiece and the laser beamrelative to one another in response to the coordinate position commandinformation; sensing an actual position of the workpiece relative to thecoordinate position command information; producing one or more errorsignals indicative of a difference between the coordinate positioncommand information and the actual position, the difference including anerror signal component representing laser beam position errors at theworkpiece; producing a position correction signal in response to eacherror signal produced; positioning a high-speed beam steering deviceadapted to receive the laser beam and, in response to the positioncorrection signal, to impart to the laser beam angular motion thatdeflects the laser beam in a manner sufficient to compensate for thelaser beam position errors, the high-speed beam steering device having abeam deflection range limited to less than or equal to 100 microns atthe workpiece; and providing a focusing lens having an entrance pupiland positioned to receive the deflected laser beam and focus it on thetarget location on the workpiece, the high-speed beam steering devicebeing within 1 mm of the entrance pupil.
 2. The method of claim 1 inwhich the high-speed beam steering system comprises a fast steeringmirror.
 3. The method of claim 1 in which the high-speed beam steeringdevice comprises a two-axis steering device.
 4. The method of claim 1 inwhich the target location comprises any one of multiple targetsemiconductor links located in a first link row and in second and thirdlink rows adjacent to the first link row, and in which positioning theworkpiece and the laser beam relative to one another further comprises:providing relative motion between the focusing lens and the workpiece ina first travel direction generally parallel to the first link row;processing at least a first target semiconductor link in the first linkrow during a first pass in the first travel direction; and employing thehigh-speed beam steering system, positioned upstream of the focusinglens, to deflect during the first pass the laser beam in a second traveldirection transverse to first travel direction to process at least asecond target semiconductor link in either the second link row or thethird link row.
 5. The method of claim 4 in which semiconductor links inthe first, second, and third link rows are generally arranged incolumns, including first and second adjacent columns, and in which thefirst and second target semiconductor links are located in the first andsecond columns.
 6. The method of claim 4 in which a total number of linkrows each contain target semiconductor links, and in which the focusinglens is scanned in the direction generally parallel to the first row anumber of times that is fewer than the total number of link rows toprocess all the target semiconductor links.
 7. The method of claim 4 inwhich semiconductor links in the first, second, and third link rows aregenerally arranged in columns, including a first column, second andthird columns that are adjacent to the first column, and at least onefourth column that is nonadjacent to the first column, and in which thefirst and second target semiconductor links are located in the first andfourth columns.
 8. The method of claim 4 in which the relative motion ofthe focusing lens in the parallel direction occurs at a travel speedduring nonprocessing periods and at a processing speed during aprocessing period including a link severing event, the processing speedbeing slower than the travel speed, and in which at least two targetsemiconductor links are processed during a single processing period. 9.The method of claim 4 in which the relative motion between the laserbeam and the workpiece along the first travel direction is imparted by amotion stage.
 10. The method of claim 1 in which the high-speed beamsteering device comprises a piezoelectric component.
 11. The method ofclaim 1 in which the high-speed beam steering device comprises aelectrostrictive actuator.
 12. The method of claim 1 in which thehigh-speed beam steering device employs a voice coil actuator.
 13. Themethod of claim 1 in which the high-speed beam steering system has amaximum beam axis deflection range of 50 microns at the workpiece. 14.The method of claim 1 in which the high-speed beam steering device has amaximum beam axis deflection range of 20 microns at the workpiece. 15.The method of claim 1, further comprising: employing a calibrationprocedure to characterize movement of the laser beam by the high-speedbeam steering device prior to processing a target location.
 16. Themethod of claim 1, further comprising: employing a command generator togenerate control signals of a motion stage and the high-speed beamsteering device.
 17. The method of claim 1 in which an error signalconveys correction information concerning a laser beam quality metricthat is affected by a disturbance of the laser beam alignment.
 18. Themethod of claim 1 in which an error signal conveys correctioninformation concerning an environmental condition, a fluctuation ofwhich alters the laser beam alignment.
 19. The method of claim 1 inwhich a pair of galvanometer-driven mirrors is employed to position thelaser beam relative to the workpiece, the pair of galvanometer-drivenmirrors being distinct from the high-speed beam steering device.
 20. Themethod of claim 19 in which the high-speed beam steering devicecomprises a piezoelectric component.
 21. The method of claim 1, in whichpositioning the workpiece is carried out at a servo cycle rate, andpositioning the high-speed beam steering device is carried out at anupdating rate that is substantially greater than the servo cycle rate.22. The method of claim 1, in which the one or more error signalsconform to a first coordinate system and the motion of the high-speedbeam steering device is characterized with reference to a secondcoordinate system, and in which the method further includes transformingat least one of the error signals into the second coordinate system. 23.The method of claim 1 in which the laser beam position errors includelinear stage ringing errors.
 24. The method of claim 1 in which thelaser beam position errors include an AOM-related error.
 25. The methodof claim 1 in which the laser beam is deflected in two axes in responseto the position correction signal to provide an undistorted focused beamspot at the workpiece.